1. Field of the Invention
The present invention relates to standardized mechanical interface (SMIF) systems for facilitating semiconductor wafer fabrication, and in particular to an adaptor plate for allowing 200 millimeter (mm) SMIF pods to be used on interface ports of wafer processing equipment sized to accept 300 mm SMIF pods.
2. Description of the Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, Jul. 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 .mu.m to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half micron (.mu.m) and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes which hold the semiconductor wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
It has been an industry standard to fabricate semiconductors on a circular wafer having a 200 mm diameter. A plurality of these wafers may be located within wafer-carrying cassettes, which cassettes are stored and transported between processing stations in the sealed SMIF pods. A typical 200 mm cassette 20 is shown in FIG. 1, and a typical 200 mm SMIF pod 22 housing the cassette 20 is shown in FIGS. 2A through 2C. The SMIF pod 22 shown in FIGS. 2A through 2C has a base with a substantially rectangular footprint with a length (corresponding to a side of the cassette) of approximately 280 mm and a width (corresponding to a front of the cassette) of approximately 267 mm. The height of the pod may vary to accommodate higher or shorter cassettes, but a typical pod may have a height of approximately 254 mm.
In order to increase capacity in semiconductor manufacturing facilities, there is presently a movement toward standardization of a 300 mm semiconductor wafer, and equipment is being redesigned to accommodate 300 mm wafers. For example, U.S. Ser. No. 08/311,954, previously incorporated by reference, discloses a circular cassette for storing and transporting a plurality of 300 mm wafers. 300 mm SMIF pods have also been designed to house the 300 mm cassettes. One type of 300 mm pod includes a base having a circular footprint to minimize the overall size of the pod.
Regardless of size, semiconductor wafers are conventionally loaded into a processing station, such as processing equipment 24 shown in FIGS. 3A and 3B, by locating a pod 26, containing the wafer-carrying cassette, on a canopy 28 on top of the processing equipment. During transport and storage of a wafer-carrying cassette, the cassette is supported within the pod 26 on a pod door 30. The pod further includes a pod cover 32 which mates with the pod door to provide a sealed environment within the pod. When the semiconductor wafers are to be transferred into the processing equipment 24, the pod is loaded onto an access port on an upper horizontal surface of the canopy 28, such that the pod door lies in contact with a port access door 34, and the pod cover lies in contact with a port plate 36 that surrounds the port access door. The pod door and port access door are shown by the dashed lines in FIG. 3B.
Typically, after location on the canopy access port, the pod cover is decoupled from the pod door, and the pod door and port access door are lowered into the processing equipment by an elevator 38. It is also known to maintain the pod door and port access door in a stationary position, and to raise the canopy and pod cover. In either embodiment, the pod door and the port access door are preferably opened at the same time so that particles which may have been on the external door surfaces are trapped, or "sandwiched" between the doors. The elevator 38 lowers the pod door and the port access door with respect to the pod cover, with the cassette riding on the pod door, into the processing equipment. The pod cover 32 remains in a secured position on top of the access port, coveting the entire access port so that no contaminants from the environment surrounding the processing equipment 24 enter the into the equipment while the port access door is retracted from its sealing position over the access port. After processing, the reverse operation takes place.
The access port of the above described system has conventionally only been able to accept a pod of a particular size, i.e., one for which the access port is configured, so that the pod cover fully and properly seals the access port while the port access door is retracted from its sealing position over the access port. Thus for example, a 200 mm pod could not be used with conventional processing equipment sized to accept a 300 mm pod, in part because the 200 mm pod cover is too small to cover the entire canopy port, and contaminants would be allowed to enter the processing equipment when the port access door was opened. While there is a movement toward standardizing the semiconductor industry to a 300 mm wafer and equipment, even if this occurs, there will be a significant period of time where it would be advantageous to provide equipment capable of handling both 200 mm wafers and 300 mm wafers.